1. Technical Field
The present invention relates generally to the field of communication electronics and, in particular, to a method for reducing frequency glitches during ramping of a power amplifier in a digital transceiver.
2. Description of the Related Art
Spread spectrum is a communication technique that has found widespread use for both military and commercial applications. In a spread spectrum communication system, the transmitted modulation is spread (i.e., increased) in bandwidth prior to transmission over the channel and then despread (i.e., decreased) in bandwidth by the same amount at the receiver.
One of the target applications for spread spectrum is to facilitate wireless or radio communications between separated electronic devices. For example, a wireless local area network (WLAN) is a flexible data communication system that uses radio technology to transmit and receive data over the air, thereby reducing or minimizing the need for wired connections. In a typical WLAN, an access point is provided by a transceiver that connects a wired network from a fixed location. End users connect to the WLAN through transceivers that are typically implemented as PC cards in a laptop computer, or ISA or PCI cards for desktop computers. The transceiver may also be integrated with any device, such as a handheld computer, personal digital assistant, or the like.
The majority of the WLAN products available in the marketplace today are proprietary spread spectrum solutions targeting vertical applications operating in the 900 MHz and 2.4 GHz ISM frequency bands. These products include, as mentioned above, wireless adapters and access points in PCMCIA, ISA and custom PC board platforms. A typical spread spectrum transceiver comprises a conventional IF radio circuit, coupled to a baseband processor, which provides the desired modulation of the signal to be transmitted and the desired demodulation of a signal received at the transceiver. The IF radio circuit includes a frequency synthesizer that includes a voltage controlled oscillator (VCO) and a phase-locked loop (PLL). The baseband processor performs a given spread spectrum modulation technique such as direct sequence (DS) modulation, frequency hopping (FH) modulation, time hopping (TH) modulation, or hybrid modulations that blend together one or more of the various schemes.
The spread spectrum transceiver as described above typically operates in a time division duplex (TDD) mode of operation wherein the transmitter is switched on during packet transmission and switched off during packet reception. The transmitter includes a number of components including a power amplifier, and a pair of up-converter mixers. Typically, the transmitter power amplifier is only turned on when sending a data packet (or perhaps just before). The power amplifier is “ramped” on (as opposed to being hard-switched) to reduce spectral splatter, i.e., the leaking of RF signals into adjacent signal channels. Power ramping is achieved by adjusting (i.e., increasing) the gain of the power amplifier.
The frequency synthesizer, on the contrary, must remain on during both TDD signal transmission and reception. When the power amplifier is ramped, however, undesirable frequency variations are produced in the frequency synthesizer due to changes in the amplifier's input impedance. The phase-locked loop cannot instantaneously correct for these frequency variations. Moreover, as the frequency error becomes large, a far end receiver cannot be synchronized properly to the transmitted signal.
In the prior art, this problem has sought to be addressed by isolating the transmitter power amplifier from the synthesizer VCO. FIG. 3 shows the additional isolation, namely the buffers 326 and 312, incorporated between the VCO 322 and the power amplifier 314. The buffers 326 and 312 are usually multistage sections in order to attain a high isolation as defined by ∘ S21i/∘ S12i, S12 being the forward gain (from point A to B) and S21 being the reverse gain. FIG. 1 illustrates the isolation process, which involves switching on the transmitter components (other than the power amplifier) at the end of a reception period Rx and then ramping the power amplifier at a later instant, usually upon transmission of a preamble that precedes the actual signal payload. As shown in FIG. 1, however, this operation still results in an undesirable VCO frequency transient (i.e., a glitch) when the power amplifier ramps on. Moreover, this isolation technique is not sufficient for fast switching transceivers that need to comply with IEEE 802.11 standards, which require frequency accuracy better than 25 ppm.
The present invention addresses this problem.